Over the past 25 years or so, a host of wireless mobile communication services have been successfully developed and deployed. Mobile communications initially provided mobile phone applications to support voice calls, e.g., along highways where landlines were not readily available. Wireless service then expanded territorial coverage into urban environments and residential neighborhood. Rapid development of wireless infrastructure and continued roll out of advanced wireless mobile technologies continues to expand access to such services. Service feature coverage has been extended from voice communications to include data applications such as text messaging, e.g., SMS, and email. Advances in wireless mobile data services eventually merged with Internet access allowing mobile devices to support online data search and to deliver streaming media, such as video clips, including video on demand and multicast of television (TV) programs.
Mobile devices are becoming more akin to mini computers, providing users with wireless access virtually anytime and anywhere, and serving as concentrated service delivery points for multi-mode and multi-band network access.
The success of mobile internet access makes an urgent call that more radio frequency (RF) spectrum is required as the basic resource to support its fast growth and higher service performance.
In general, there are two popular access technologies in the field of mobile Internet. One is referred to as Frequency Division Duplex (FDD) and the other is referred to as Time Division Duplex (TDD). FDD access technologies assign one spectral segment, or block of RF spectrum, for the downlink communications, e.g., from a base station to a mobile device, and another block of RF spectrum for corresponding uplink communications, e.g., from the mobile device to the base station. A so called guard band is generally provided, e.g., by regulation, between the uplink and downlink blocks of RF spectrum, providing some spectral separation between the two bands to avoid any possibility of interference. Once an FDD session, e.g., an uplink and downlink channel is assigned, no matter the uplink or downlink transmitter is transmitting or not, its spectrum is already allocated and no other session can use it.
TDD access technologies are distinguishable from FDD access technologies. Instead of pairing allocated spectrum as in FDD, there is only one RF block assigned to communication sessions, including both uplink and downlink transmissions. Spectrum sharing is accomplished in the time domain, such that at any time, there is only one-way transmission within the given RF block, an uplink or a downlink, but not both uplink and downlink. This is true no matter how many wireless devices are being served within the same RF block and by the same base station.